Electrical measuring method and means



Sept. 2. 1924. 1,506,895

F. E. FIELD ELECTRICAL MEASURING METHOD AND MEANS Filed April 15. 1922 hue/7 for.-

Sept. 2, 1924.

UNITED STATES PATENTOFFICE.

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A uuuonmd s in 15, 1022. Serial llo. 559,870.

Toallwhomz'tmayconcem:

Be it known that I,Fns1-tx E. Fmm, a citizen of the United States, residing at Somerville, in the county of Somerset, State of New Jerse have' invented certain new and useful provements in Electrical Measuring Methods and Means, of which the following is a full, clear, concise, and exact description. V V

This invention relates to the art of electrical measurement and particularly to a method of and means for determining the transmission loss of electrical apparatus to be connected between circuits of different impedances. The invention is set forth hereinafter with especial reference to its application to the determination of the transmission loss of telephone repeating coils or other transformers, but is applicable to electrical apparatus-in general.

The invention contem lates the testing of apparatus under consitions simulating actual service conditions for the apparatus; to eliminate errors due to variation of supp113;1 voltage; to avoid dilficulty' and error in ta 'ng readi due'to external disturbances during the ta ing of the readingsyto obviate an necessity for calibrating amplifiers emp oyed in making the measurements and to avoid cha ing the impedance o the test circuit during. .the test. 1

In .accordance with g the' invention, as applied to the measmeniehtof the transmisn M sts a the coil is l f in a circuit such that service conditions "for the coil are simulated.

An alternating current is then impressed on the coil, the transmission loss of the coil isfid etermined from theiratio of the value. of the resulting secondary current to thevalue that the secondary current would have'ifthe transformer were perfect-that is,-had'in1inite mutual inductance, zero direct current resistance, zero leakage, and an impedance ratio equal the ratio of the im- P ances of the cif-cuits coupled by the transformer; Thus, the transmission loss of the transformer-here indicates the differeneefiibetween the transmission gain which would result from employing a perfeet transformer instead of notransformer and the transmission gain which results from employing the actual transformer instead of no transformer. In other words, the ton ting coil or transformenfi loss of the actual trans former is here considered as represented by the sin in the transmission efliciency of the clrcuit which would result from replacing the actual transformer by a perfect transformer.

The drawing illustrates the invention applied to the measurement ofthe transmission loss of'a repeatingcoil. 3

Energ is supplied to the low voltage winding of the repeating coil 1 from an 08- cillator O or other suitable source, ably through a transformer 2, whi

ma be a shiel y ed transformer, as indicated on the drawing in order to preventinterfer-- ence from external sources; R and R" are resistances for re lati the voltage applied to the trans ormer During all measurements, when looking from the coil 1 in either direction, one should see impedance values corresponding to the l liarticular service conditions for which t e transmission equivalent of the coil is to be'determined. Therefore, the high voltage side of coil 1 is connected to non-inductive resistance condenser or other appropriate an impedance Z,, consisting of the proper.

and r, and

regards the valueand phase angle of the imgfdance "connected to the secondary or big voltage side of coil ,1. -Similarly, an

reactance element C to slmulate servlce conditions as-- referim edance'Z consisting of non-inductivev resistance R, and condenser or other proper reactance element 0,, is connected in cir-= cuit with the low sideof coil 1 to simulate servicefconditions, a non-inductive resistance rqbeing connected across R,, C, and the pr maalfi of the coil and made of negliibly 'sm im edance in comparison to iatof R, and Thus and-iii"; ual tothe impedance into which the low side of the coil 1. is desired to work in'servicet; Also, r l

e of coil is desired to work nected in parallel with 1-,, Z being made equal to where K is the impedance ratio of the transformer, that is, Z being made equal to the impedance which the circuit to the right of 1', would resent if. the transformer were perfect. A t ermocouple or the like is placed in series in 0",, in order to measure the current through 1-, and thus facilitate the adjustment of the current throu h Z, to a desired value. The resistance of t e thermocou Is is included in 7-,.

eans is provided for balancing or comparing the voltage drop across r, and the voltage drop across 7",. For instance, an indicating galvanometer 3, or the like, and a double throw switch 4, or the like, may be used to measure the voltage drop across r, and the voltage drop across. 1- in quick succession. Preferably a three stage vacuum tube amplifier or the like is interposed between the switch 4 and the instrument 3 and the vacuum tube rectifier or other appropriate means is connected to the am lifier so that a direct current micro-a eter may be used as the indicating galvanometer. The am lifier and rectifier are designated 5 in the rawing.

In operation, 7", is set at some convenient value and 1- is varied until the voltage drop across '1", is equal to the voltage drop across 13, these drops being compared by quickly throwing the switch 4 from one of these resistances to the other. At all times during this balancing of the circuit Z and Z must be kept at the proper predetermined values. When the same reading is obtained on the micro-ammeter for the voltage drop across 1', as is obtained for the voltage drop across 1*, the desired current ratio, from which the transmission equivalent of the transformer 1 may be obtained in a manner indicated hereinafter, is equal to the ratio multiplied by the voltage ratio When the circuit is balanced 'r I zqqI y from which L-Ej (2) Substituting (2) in (1), we have -afs l. e)

Let

Since, as is indicated above, the impedance of 1' is low, the value of E would not be appreciably dilferent from the value of E, and therefore, we may equate (4) and (3), glvlng placed by a perfect coil.

In equation (6) fis the current ratio determining the transmisv is the ratio of the resistances the dro s across which are balanced to obtain a rea ing. The

desired for sion equivalent of the coil and ratio 2 obtaining under the balance condition multiplied by {K will equal I Z 1 z K is made equal to unity, that is if =(Z fi);

but Z, actuall doeshave this value, as was pointedout a ove. Therefore,

4: T 1 (a, m This current ratio, which we may designate R, is a measure of the transmissitmloss of the transformer, and may be exprem in chosen frequency. As explained on ages 848 and 862, vol. 2 of Sir Richard lazebrooks Dictionary of Applied Physics, MacMillan Company, New York, 1922, the relation expressing the ratio R in terms of units of length of standard cable is;

a 2.3026 log units of length of standard cable where a is the attenuation constant per unit,

of length of standard cable at the chosen freuency. The value of a is proportional to t e square of the frequency. Usually, the frequency chosen is 796 cycles per second,

miles of standard cable= The miles of standard cables, given by this equation, is designated the transmission loss (or loss, in miles of standard cable, at 796 cycles corresponding to the ratio R. This loss is the transmission loss of the transformer in the desired terms it being understood that, as noted above, t e transmission loss in the circuit due to employment of the actual transformer instead of a perfect transformer is regarded as the transmission loss of the actual transformer.) Of course, from the current ratio R determined as described above, the transmission loss of the transformer may, if desired, be obtained in terms other than miles of standard cable.

R 7-,, and R,, r, may be graduated in terms of resistance, of course. Or, since where k is a constant for a gjzven setting of r, for a given transformer impedance ratio, R 7', can be so graduated that under these conditions the reading of P will give the .value of the ratio directly. Such graduation may be very desirable where a number of transformers havin the same impedance ratio are to be teste Since the attenuation coeflicient a, is constant for a fixed frequency, the impedance R, 1' can also be so graduated that for a fixed frequency the reading of 'r will give the loss'in the transformer directl in terms of the number of miles of standard cable which would have the same loss.

Although it has been stated above that 2.3026 log and the unit of length chosen is a mile. The value of a is then .109, and the relation expressing R in terms of units of length of standard cables becomes:

=21.12 log it may be seen from equation (6) above that Z can, instead, be made equal to in which case and the current through Z will be equal to I instead of to I The current ratios which have been obtained by means of the circuit of this invention in the determination of the transmission equivalents of various apparatus a ree closely with the calculated values of ese ratios. For instance, in measuring the transmission loss of several different receiver shunts each consisting of a series and a shunt resistance, the readings obtained checked the calculated values within less than .5% for losses varying from 1 to 30 miles.

With the clrcuit herein described an ordinary change in voltage of the oscil ator will not affect the balance obtained, since both of the voltage drops 1-, I and r I vary with the supply voltage and these voltage dro s are measured substantially simultaneous It? practicing this invention the amplifier need not be calibrated since it is only used as an element of an indicator and not as a measuring device.

Since in the circuit here disclosed no highly sensitive alternatin current measurmg device is required, an the direct current meter employed is fed through an amplifier-rectifier device, which does not have to be calibrated, an necessity for careful calibration and ad ustment of a ourcurrent therefronri 1 rent-measuring device is obviated, and trouble due to external disturbances during the taking of readings is avoided.

In the system disclosed herein the impedance of the circuit is not appreciably changed during the test, for theinput impedance of the amplifier-is so high that its shunting efl'ect is negligible.

In accordance wi this invention when measurements are taken on a repeatingcoil or transformer, thecixcuit im ances may be made to correspond to e impedance ratio of the soil It isdesiruble to measure the efiiciency of the coil underthe condition that the circuit impedance ratio and the impedance ratio of the coil are equal, for the. reason that this is the condition for best results, or highest current output front the coil.

What is claimed is: V

1. The method of determining the cm ped mission loss of electrical apparatus to be connected between circuits of different impedances, which comprises supplying input current to the apparatus to produce output reducing a third current of a value erent from said input current and bearin a predetermined relation to the value t at said output current would have if the transmission equivalent of the apparatus were zero, and physically compar the values of said output current and said ird current.

2. The method of determining the transmission lcss of a transformer which comprises producing output current therefrom, producmg a current of a value bearing a predetermined relation to the value that said outputcurrent would have if the transformer were rfec and hysically comparing the va nos 0 said rat and second mentioned currents.

8. The method of determining the transmission loss of a transformer which comprises producing an output current therefrom by a plying an electromotive force to said trans ormer under conditions simulating its service conditions, producing a voltage drop from said output current, producing another current from said electromotive force, producing a voltage drop, from said other current under predetermined conditions such that when said drops are, equal the ratio of the uotient of one of said drops divided by t a current producing it to the quotient of the. other 0 divided bv the current producing it bears said drops 1 Lucas a predetermined relation to the ratio of the va output current from said transformer means for producing a third current of a va ue different from said influt current and bearing.

tion to the value that a known simple re said output current would have if the transformer were a' rfect transformerv and current an said output current.

5. In combination, to source of electromotive force, a transformer, impedance elements, and means for obtaining two equal voltage drops, one from one of said imance elements and one from another of said impedance elements and for determining the relative values of the im ances across which said drops occur, sai source, said transformer, and all of said impedance elements bein so roportioned and arranged that t a ratio of said impedance values bears a known simple relation to the ratio of the value of the out at current from said transformer to the va ue that the output current would have were the transformer a perfect transformer.

6. n combination, a circuit comprising a transformer having an in ut winding an an out at win ,an imp ance connected to sai output winding, and an impedance connected to said input winding, a source of volta connected across the second man tione impedance andisaid' input winding,

animpedanceconne'cted across said source,

means .for obtaini a voltage dro across at least a part of sai first mentions impedance and for obtaining an eiual voltage drop, across at least a part 0 said third mentioned impedance, said, third mentioned in once having a value such that the impe ance ratio of said parts corresponding to said drops bears a known simple relation to the ratio of the value of the current in said first mentioned impedance to the value said current would have were said transformer a cot transformer.

witness whereof, I hereunto subscribe means forfhysica y comparing said third my name this 13th day of April, A. D.

ream; a. FIELD. 

